PURPOSE: To safely and accurately build a structure such as a bridge with the use of more than three floating cranes even though an unexpected accident occurs, by centrally managing the balanced conditions of hook loads of all floating cranes on a real-time base.

CONSTITUTION: A suspended load 20 is detected by a load meter provided on each of cranes 4 of floating crane assemblies 2, and delivers its output to a personal computer 23 through a d.c. amplifier 21 and a data logger 22. The computer 23 transmits its output.signal to a radio modem 24' in a work instructing station 17 from an associated radio modem 24 by way of radio-waves 12. Further, data concerning received signals are delivered to only one personal computer 23', and after a predetermined computing process is carried out, the data are fed back to ship control parts of the floating crane assemblies 2 through the modems 24, 24' so as to carry out a predetermined ship control. That is, the ship is controlled so that hook loads of a bridge at the cranes are hung up and down with an optimum balance, and simultaneously, the hook loads in a controlled is balanced condition are displayed on a CRT for every floating conditions.